Method of forming an engine valve of a ferrous metal containing chromium and nickel by heating treating and deforming



United States Patent METHOD OF FORMING AN ENGINE VALVE OF A FERROUSMETAL CONTAINING CHROMIUM AND NICKEL BY HEATING TREATING AND DEFORMINGRichard A. Kloske and William F. Barclay, Parma Heights, Ohio, assignorsto Republic Steel Corporation, Cleveland, Ohio, a corporation of NewJersey No Drawing. Continuation-impart of application Ser. No. 607,406,Jan. 5, 1967. This application Nov. 29, 1967, Ser. No. 692,618

Int. Cl. CZld 7/02 US. Cl. 14812 3 Claims ABSTRACT OF THE DISCLOSURECold formable valve steel of composition 1923% chromium, 40-65% nickel,6.28.5% manganese, 0- 1.5% silicon, 0.15-0.3% carbon, 0.15.0.3%nitrogen, 0-0.2% columbium and/or tantalum, 0-0.l% phosphorus, 0-0.1%sulphur, balance substantially iron, articles thereof and production.The above described alloy is utilized in making an engine poppet valve.A part made of the alloy is solution treated and quenched. Subsequentlythe part may be reheated for working. It is then extruded and upset toform a valve shape.

This application is a continuation-in-part of our copending applicationSer. No. 607,406, filed Jan. 5, 1967.

This invention pertains to steels especially adapted for use in exhaustvalves for internal combustion engines, and provides a steel of novelcomposition and improved properties therefor, which is furthercharacterized over steels which are presently commercially acceptablefor such applications, in being cold-formable, as by upsetting andextruding, into valve configuration. The invention also pertains toarticles made of said steel and the production thereof.

The steel of the present invention is particularly adapted for use inexhaust valves for automobile and other internal combustion engines, therequired combination of room and elevated temperature minimum propertiesfor which are particularly severe. Typical minimum physical propertyrequirements for a cold-formable steel for such applications are asfollows: The elevated temperature rupture strength as measured by stressfor 1% stretch at 1350 F. in 100 hours, should be 6000 psi. minimum, andas measured by stress for rupture at 1350 F. in 100 hours should be10,000 psi minimum. As regards hardness, that at 1400 F. should be atleast 90 Brinell, and that at room temperature should be at least 27Rockwell C. The oxidation resistance as measured by weight loss in gramsper sq. dm. per hour (g./dm. /h.) should not exceed 60, as determined byheating a specimen in molten lead oxide, Pb O at 1675" F. The roomtemperature impact strength as determined by the Charpy V-notch testshould be at least 5 foot-pounds. And the cold-formability should becomparable to that of type 305 stainless steel (18Cr-12Ni0.12C).

The steel of the present invention not only has adequatecold-formability, but exceeds these minimum physical propertyrequirements in every respect as shown by the test results hereinafterpresented.

The steel of the invention is essentially a substantially austenitic,medium carbon, high nitrogen, chromium- Patented Dec. 22, 1970nickel-manganese steel of the following broad and preferred compositionranges.

AMO UNT-WEIGHT PERCENT Balance, Substantially Fe.

1 Maximum.

A preferred specific composition is of substantially the analysis: 21%Cr5% Ni-7% Mn0.2% C0.2% N- (0-0.1% Cb-Fe. The steel of the inventionpreferably contains columbium in amount of about 0.05 to 0.2%, althoughtantalum may be substituted for part or all of the columbium.

The steel of the invention is of a balanced composition which isextremely critical with respect to the limits for each of the essentialelements above specified. That is to say, our investigations have shownthat the coldformability and other property requirements above stated,are obtained by so balancing the composition that the steel iscompletely or substantially austenitic at room temperature. Ourinvestigations have further established that the criticalstructure-property relationships of the steel can be obtained only bycarefully balancing the Ni, Mn, C and N values of the austenite.

The chromium content is set within critical limits of 19-23%, to assureadequate scale resistance and secondary hardening at service operatingtemperatures up to about 1400 F. When aged at about 13001400 F. thesteel undergoes secondary or age-hardening by precititation of carbidesand nitrides, and also phosphides if the steel contains an appreciableamount of phosphorus, i.e., in excess of about 0.04%. The sulphurcontent of the steel should not exceed about 0.1% and preferably notexceed 0.04%.

Silicon, which is usually present as a residual element in these steels,may be employed in amounts up to about 1.5%, the preferred upper limitbeing about 1%.

While boron is not an essential constituent of these steels, it may betolerated in residual amounts up to about 0.008%. p

The nickel content should be maintained as high as is commerciallyfeasible to insure cold-formability. Our investigations indicate thatthe lower limit for nickel content on the basis of cold-formabilityrequirements occurs at about 4%. Also that such increase incoldformability as is obtained with nickel contents in excess of 66.5%,is insufiicient to warrant the increase in alloy costs.

Neither high nickel nor nickel plus manganese is employed or required inapplicants steel for imparting an austenitic structure, this beingachieved more efficiently and cheaply by the relatively high contents ofthe interstitials, carbon and nitrogen. Thus applicants steel differsfundamentally in this respect from low carbon austenitic steels of, forexample, 0.05% max. carbon, which are strengthened exclusively bynitrogen additions, and which require a minimum of about 14% nickel plusmanganese for imparting a fully austenitic structure, and also a minimumof about 8% manganese for preventing ingot porosity. In applicants steelmanganese is employed primarily for maintaining a high nitrogen contentin the steel as an austenitizing agent. For applicants steel to containthe above specified amount of about O.l50.3% nitrogen, manganese isrequired in minimum of about 5% up to not more and 7.5% depending on theparticular analysis.

As above stated, austenits stability is achieved in applicants steelprimarily by the interstitial additions, carbon and nitrogen. However,since progressively increasing additions of these elements rapidlystrengthen the austenite, thereby correspondingly reducing thecoldformability of the steel, it should preferably contain justsufiicient carbon plus nitrogen to maintain a fully austeniticstructure.

In order to demonstrate the properties of steels according to theinvention with respect to the properties above discussed, compositionsaccording to the following Table I were melted and thereafter tested asdiscussed below.

TABLE I.NOMINAL AND ANALYZED COMPOSITIONS OF THE EXPERIMENTAL COLDFORMING VALVE STEELS [21-5-7/Cr-Ni-Mn steels: varying C, N, P, Obcontent] Alloy content (weight percent) I-l'cat Compo- No. sition Cr NiMn Si O N 1 Cb B40 {NominaL 21. 5.0 7. 0 0. 0.20 AetuaL 21. 35 5. 35 6.55 0. 49 0. 194 B51 {Nommalu 2. 5. 0 7. 0 0. 5 0. ActuaL 21. 80 5. l0 6.50 0. 66 0. 197 B 41 {NominaL 21. 0 5. 0 7. 0 0. 5 0. 4O Actual. 21. 65. 10 6. 9O 0. 51 0. 37 B52 {Nominaln 21. 0 5. 0 7. 0 0. 5 0. A0tual 21.5. O0 6. 55 0. 59 0. 289 B53 {NomiuaL 21. 0 5. 0 7. 0 0. 5 0.20 Actual.21. 8O 5. 15 6. 80 0. 60 0. 22 B54 {NominaL 21.0 5. 0 7. 0 O. 5 0. 20Actual 22. 30 5. 15 6. 70 0. 49 0. 20 B42 {N0rninal 21. 0 5. 0 7. 0 0. 50. 15 Actual 21.7 5.15 6.90 0.56 0.136 0.116

Specimens of each of the above steels were tested for hardness, creepstrain and impact strength in various heat-treated and otherwiseprocessed conditions, with results as s hown in the following Table II.

4 additions of phosphorus and columbium, the test data clearlydemonstrates the critical effects of the interstitial contents.

Room temperature tensile properties of the aboveexemplified steelsaccording to the invention in the condition as solution treated at about2100 F. and water quenched are shown in the following Table III.

TABLE III.IENSILE PROPERTIES OF NOMINAL 21 Cr- 5N1 7Ml1 STEEL WITHVARYING C, N, P, Ob

Heat Alloy content 0.2% Y.S. U.T.S. El, RA, N 0. (wt. percent) K s.i. Ks.i. percent percent Machined tensile specimens for the above tests Wereground to 0.505-inch in diameter and a 2-inch gauge length was employedfor measuring elongation.

From the Table III data it will be seen that all of the steelsexemplified have in the as-annealed, i.e., solutiontreated and quenchedcondition, relatively low yield strengths and high ductilities, thelatter as measured both by elongation and area reduction, conductive togood coldforming properties. From this data it will further be seen thatsteels according to the invention, as solution treated and quenched,have 0.2% otfset yield strengths of not more than about 75 k.s.i.,together with tensile elongations in excess of about 35% and areareductions of more than about It is evident from this data that thesesteels are characterized by a high degree of plastic deformability atroom temperature. As shown by Table II, all were reduced by cold rollingwithout difficulty. However, for cold upsetting and extrusion intointernal combustion engine valves, the steels having area reductions ofat least about 60% with low yield strengths were found most suitable asclosely approximating to cold-formability of type 305 stainless steel,the annealed properties of which are TABLE II.MECHANICAL PROPERTIES OFTHE EXPERIMENTAL COLD FORMING VALVE STEELS [21-5-7/Cr-Ni-Mn steels;varying C, N, P, Cb content] Creep Hardness Rockwell C strain 2 1,350F., 1,400 F. Cold Aged 10,000 p.s.i., Brinell 70 F, rolled (at 1,350 F.)percent Hardimpact Heat Nominal alloy Solution Ground strain/test nessstrength No. content, wt. percent treated 1 surface reduction 100 hr.1,000 hr. duration HN 4 ft. lbs.

B40 0. 2C, 0. 1P 12. 3 15. 4 41. 8 39. 8 38. 3 17. 0/75 hr 95. 6 4 B510. 2C, 0. 2N, 0. 1 12. 1 15. 6 44. 6 40. 0 38. 4 0 57/100 hr 124. 5 4B41 0. 4C, 23. 5 26. 5 48. 2 34. 9 33. 5 24 2/57 hr 108. 9 17 B52 0. 3C,0 IN 17.1 19.4 45. 4 34. 8 34. 6 24 3/93 hr 97. 2 9 B53 0. 2C, 0. 2N 12.4 13. 4 44. 9 37. 9 35. 7 0 29/100 hr 128. 3 8 B54 0. 2C, 0. 2N, 0.0701) 16.0 20. 8 45. 1 37. 5 35. 7 0 15/100111 s- 135. 4 8 B42 0.15C, 0.1N12. 9 14. 0 39. 9 37. 7 37. 4 19.2/26 hr 97.2 4

1 Solution treatment: 1 hr. at 2,050 E, Water quenched.

2 Creep sample thermo-mechanical history: solutlon treated as in 1above, ground to 0.750-inhistory: same as 1 above, aged 98 hrs. at 1,350F.

2 passes, stress relief annealed 2 hrs. at 1,350 F.

3 Hardness, impact sample thenno-mechanieal 4 BHN, 1,000 kg. load, 10mm. dia. chromium carbide indenter.

From the above data it will be seen that all of the above steels meetmost of the above-stated minimum requirements for the ideal steel forinternal combustion engine exhaust valves. Steels B53 and B54 meet allof the requirements as regards minimum room and elevated temperaturehardness, creep strain and impact strength. It will be observed thatthese two steels had aged room temperature hardnesses in the range of35-38 Rc(Rockwell C), well in excess of the specified minimum of 27 Rc.It will further be noted that these steels had by far the lowest creepstrain values of 0.29 and 0.15%, respectively, far below the specifiedminimum of 1%. In addition, steels B53 and B54 had by far the highest1400 F. hardness values of 128 and 135 BHN, well above the specifiedminimum of 90 BHN.

Since all of these steels were melted to a nominal composition of 21%Cl5%, Ni-7% Mn with varying amounts of carbon and nitrogen plus in someinstances optional dia., cold-rolled to 0.5-iu. thickness in 37,000p.s.i. yield and 85,000 p.s.i. tensile strength, with 55% elongation in2 inches and 65% area reduction. As shown by Table III the steels mostclosely approximating these values as regards low yield strength andhigh area reduction values are steels B42, 1351, B53 and B54.

Although the steels of the invention are cold-formable into valves atroom temperature, the required forming pressures are greatly reduced bypreheating to about 450 1600 F., or more generally below the temperatureat which recrystallization would otherwise occur during forming, thepreferred temperature range being below about 1280 F. This mightproperly be termed warm forming in contrast to the conventional hotforming of valve steels at a forging heat of about 19002200 F.

Steels according to the invention are preferably produced by melting theingredients in the electric arc furnace, teeming in a molten state intoladles, following by casting into ingot molds. The stripped ingots arereheated to about 1900-2200 F., and for va ve applications, hot rolledinto bars and air-cooled to room temperature. For cold or warmextrusion, the bars are solution-treated at 2050-2150 F., preferably2100" F., usually for about one hour or until all carbides are insolution, and thereupon cooled to room temperature with suflicientrapidity, as by water quenching, to retain carbides in solution, vinwhich state the hardness is about -20 Rc. The bar stock is then cut intolengths for valve components and cold or warm upset into valve shapes,the resultant hardness being about 40-50 Re. The valves are thenstress-relieved at about 1300-1400 F., preferably at 1350 F.,for abouttwo hours and air cooled to room temperature, the resultant hardnessbeing at least 35 Rc. The valves are placed in service in internalcombustion engines having a service temperature for exhaust valves ofabout 1350 F. While in service the valves undergo secondary or agehardening such as to maintain the room temperature hardness above 30 Rc.

With the exception of the ability to be cold-formed into a finishedpiece, the most severe mechanical property requirement a cold-formingvalve steel must satisfy, is the ability to resist further deformationby creep at the internal service temperature. The major alloy designproblem is economically to utilize the residual strengthening resultingfrom the forming operation together with precipitation hardening at theservice temperature to transform a steel having good cold-formingcharacteristics at lower temperatures into one which will resistdeformation at an intermediate temperature. To achieve such atransformation in properties, a favorable structural revision must occurat the service temperature. As the following discussion will show, thecarbon and nitrogen contents of the applicants steel critically affectsthe microstructural changes accompanying aging and, hence, themechanical properties of the aged steel.

At a carbon content in excess of 0.3%, much of the carbide phase of thesteel remains undissolved after the solution treatment anneal. On agingat 1350 F. after cold reduction, the dissolved carbon precipitates atslip bands and as large globular particles randomly dispersed throughoutthe austenite. These modes of prepicitation impart resistance neither tosoftening nor to deformation by creep at the service temperature. Thus,ahigh concentration of undissolved carbides leads to an ineffectiveprecipitate structure.

A total carbon plus nitrogen content of less than about 0.3% isinsufficient to maintain a completelyiaustenitic structure in thesesteels. Consequently, some delta ferrite is present in theirmicrostructure after solution Itreatment. During aging aftercold-forming, carbides precipitate prefenentially within these ferritebands leaving e austenite matrix relatively depleted of precipitateparticles. Although this banding of the precipitate phase results in ahigh hardness which is stable at 1350 F., it presents no obstacle todeformation by creep in the precipitate-poor austenite.

0n the other hand, a solution treated steel of the invention containingabout 0.2% each of carbon and nitrogen, is found to completely dissolvethe carbide phase without introducing large amounts of high temperatureferrite.

When tested for weight loss due to oxidation, in molten lead oxide at1675 F., steels B40, B41, B54 and B42 were found to meet the requiredcondition above stated of a weight loss of less than 60 g./dm. /hr. Asshown by this and the other conditions above stated, steel B54 meets allof the requirements for valve steel application.

A good commercial melting range for steel according to the invention isabout: 0.180.28% carbon, 0.15- 0.30% nitrogen, 0-1.5% silicon, 20-22%chromium, 45-65% nickel, 6.58.5% manganese and ODS-0.20% columbiumand/or tantalum, columbium being preferred.

What is claimed is:

1. The method of producing an internal combustion engine poppet valvefrom a steel consisting essentially of about: 19 to 23% chromium, 4 to6.5% nickel, 6.2 to 8.5% manganese, 0.15 to 0.3% each of carbon andnitrogen, up to 1.5% silicon, up to 0.2% of an element of the groupcolumbium and tantalum and combinations thereof, up to 0.1% each ofsulphur and phosphorus, up to 0.008 boron, balance substantially iron,which comprises: solution treating said steel at temperaturesufiiciently high to place all carbides in solution and cooling thenceto ambient temperature at a rate sufiiciently rapid to retain saidcarbides in solution, thence reheating said steel to temperature belowits recrystallization temperature and extruding and upsetting the metalinto the shape of said valve.

2. The method of producing an internal combustion engine poppet valvefrom a steel consisting essentially of about: 19 to 23% chromium, 4 to6.5% nickel, 6.2 to 8.5% manganese, 0.15Lto 0.3% each of carbon andnitrogen, up to 1.5% silicon, up to 0.2% of an element of the groupcolumbium and tantalum and combinations thereof, up to 0.1% each ofsulphur and phosphorus, up to 0.008 boron, balance substantially iron,which comprises: hot rolling said steel into bar stock, solutiontreating and quenching said bar stock to retain all carbides insolution, cutting said bar stock into lengths suitable for forming intosaid valves, reheating said lengths to temperature below therecrystallization temperature of the steel, and thence extruding andupsetting said lengths into said valves.

3. The method of producing internal combustion engine poppet valveswhich comprises: hot rolling into bars a steel consisting essentially of19-23% chromium, 4- 6.5% nickel, 6.2-8.5% manganese, 0.15-0.3% each ofcarbon and nitrogen, up to 1.5 silicon, up to 0.1% each of sulphur andphosphorus, balance substantially iron, solution treating said bars andcutting to bar lengths suitable for forming into said valves, reheatingsaid bar lengths to temperature below the recrystallization temperatureof said steel, and thence extruding in part and upsetting in part saidbar lengths into the shape of said valves.

References Cited UNITED STATES PATENTS 3,311,511 3/1967 Goller 148-123,376,780. 4/1968 Tanczyn 14812 3,461,001 8/1969 Kuberg 14-12 HYLANDBIZOT, Primary Examiner

